The present invention relates to a novel styrene-based resin composition. More particularly, the invention relates to a rubber-modified styrene-based resin composition composed of a styrene-based resin forming the matrix phase and a diene-based rubbery polymer as the discrete phase dispersed in the matrix and capable of giving a molded article having excellent surface gloss and a high impact strength or, in particular, planar impact strength to he suitable as a material of parts of office-automation instruments and household electric appliances as well as sheets and the like.
It is a widely practiced technology that, with an object to improve the impact strength of a styrene-based resin, e.g., polystyrene, that the resin is blended with a rubbery polymer or styrene monomer is polymerized in the presence of a rubbery polymer so as to effect partial graft polymerization of the monomer on to the molecules of the rubbery polymer, the remainder of the monomer being polymerized into homopolymeric polystyrene, so that the resulting polymeric product is a so-called HI-grade polystyrene or a rubber-modified polystyrene-based resin composition which is a composite of a polystyrene and a graft copolymer of styrene on the molecules of the rubbery polymer.
The rubbery polymer in such a rubber-modified polystyrene resin composition is usually dispersed in the matrix of the styrene-based resin forming the dispersed phase. It is well known that the impact strength, rigidity and surface gloss of a shaped article prepared from the resin composition is greatly influenced by the particle size of the thus dispersed particles. For example, a decrease in the particle size of the dispersed rubbery polymer results, as a trend, in an improvement in the rigidity and surface gloss while the impact strength is rather decreased. Thus, substantially no improvement can be obtained in the impact strength of the molded articles when the particle size of the rubbery polymer is smaller than a certain value.
Accordingly, the dispersed particles of the rubbery polymer in conventional rubber-modified polystyrene resin compositions have a particle diameter of at least 1 .mu.m or, usually, in the range from 1 to 10 .mu.m. Such a particle diameter is somewhat larger than the optimum particle diameter which would give the most remarkable improvement in the rigidity and surface gloss of the molded articles so that the resin composition is under limitations relative to the fields of application.
Various attempts and proposals have been recently made with an object to obtain a good balance between the impact strength and surface gloss in the molded articles of rubber-modified polystyrene resin compositions. For example, German Patent Laid-Open Publication No. 3345377 discloses a molding material based on a high-impact polystyrene containing particles of a rubbery polymer in a controlled particle diameter and structure. Further, Japanese Patent Kokai No. 63-199717 and No. 63-207803 disclose rubber-modified styrene-based copolymers in which the type of the rubbery polymer is specified and the particle diameter of the dispersed particles and the grafting ratio on the rubbery polymer are controlled.
Although these prior art methods are not without improvements in the balance of the impact strength and surface gloss of the molded articles of the molding materials or copolymers, the improvements thus far obtained are far from satisfactory so that it is eagerly desired to develop a styrene-based resin composition of high-impact grade capable of giving molded articles having well balanced impact strength and surface gloss.